Don Shepperd is a retired U.S. Air Force major general and a military analyst for CNN.

Update: Heavy U.S. bombing jolted Kandahar, the Taliban's stronghold in Afghanistan, overnight, creating numerous, loud explosions. Secondary blasts from bombed ammunition stockpiles are believed to have propelled unspent ammunition into other parts of the city and into an Afghan refugee camp. The strikes on bunkers may indicate that the United States is amassing better intelligence about the location of Afghan troops and weaponry.

Impact: This means it's entering a new phase ... in preparation for perhaps the ground forces coming in. It's becoming clearer that ground forces are going to be introduced somehow.

Strategy: Here's my take. These were evidently bigger bombs. These are the 5,000-pound bunker-buster bombs. It's a GBU-28. It stands for guided bomb unit 28. It's a laser-guided bomb with a special fuse. It's a very tough, hard bomb that buries itself deep and then goes off underground. You keep both (personnel and artillery) underground. They know if you store it on the surface, you'll be able to see it and be able to get it. A lot of stuff is stored underground there and we're going after that too.

Tactics: We have air supremacy. At high altitudes, we can operate anywhere we want at will. It doesn't mean they don't have missiles left or anti-aircraft. But now the next challenge is being able to operate at low altitude, and those shoulder-fired missiles and anti-aircraft artillery are big threats to low-flying airplanes as we start bringing in helicopters and lower flyers.

I think the message of this is we've been watching and listening on the ground and in the air and from space of where you are. We know it, and now we're coming after you in your bunkers. It is not psychological. We are after targets that are buried in the ground. We've taken care of the air defenses first. We've taken care of the known sites on the surface, and now we're going underneath so that the message is, "There's no safety for you if we can find you."

U.S. Army Gen. Wesley Clark (ret.), a former NATO supreme commander, U.S. Army Maj. Gen. David Grange (ret.) and Air Force Maj. Gen. Don Shepperd (ret.) are serving as CNN military analysts during the war against terror. Their briefings will appear daily on CNN.com.

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